Optimisation of matrix deposition for ink analysis and molecular fingerprinting in questioned document examinations using MALDI MSI

In this study, we have explored the capabilities of Matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI-MSI) as a powerful tool for Questioned Document Examination (QDE) which can offer crucial forensic chemical insights where optical based conventional methods may be inconclusive. However, successful MALDI MSI analysis of complex questioned document samples, such as those containing overlapping inks and/or fingerprints, is strongly dependent on the matrix deposition method, which influences ionisation efficiency, reproducibility, and spatial resolution. This study presents a systematic optimisation of matrix deposition, using sublimation, to enhance the simultaneous analysis of inks and fingerprints deposited on paper. Parameters such as matrix quantity, sublimation duration, and the choice of recrystallisation solvent have been optimised using controlled samples comprised of printed inks, ballpoint pen inks, and natural latent fingerprints. The optimised sublimation protocol was benchmarked against two established matrix spraying methodologies on a mock forged document. The method enabled the successful differentiation of two optically similar, but chemically distinct, ballpoint pen inks used to alter a date on a document, while concurrently imaging latent fingerprints to provide valuable biometric and chemical intelligence relating to two separate fingerprint donors. This work establishes a validated framework for achieving reliable, high-resolution molecular analysis in complex QDE scenarios, strengthening the analytical power and reliability of MALDI-MSI in forensic investigations.